Capacity maximization for a unicast spot beam satellite system

ABSTRACT

Methods, systems, and apparatuses are presented for improved satellite communications. The satellite system may comprises at least one gateway, a satellite in orbit configured to communicate with the at least one gateway and provide a plurality of spot beams, and a plurality of subscriber terminals. The spot beams may include a first spot beam to illuminate a first region and a second spot beam to illuminate a second region adjacent to and overlapping with the first region. The first spot beam as sent to at least one subscriber terminal may be affected by (1) interference from other signal sources including the second spot beam at a signal-to-interference ratio C/I and (2) noise at a signal-to-noise ratio C/N. Reception of signals from the first spot beam by the at least one of the first plurality of subscriber terminals may be interference-dominated such that C/I is less than C/N.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of, and claims priority to, U.S.patent application Ser. No. 13/591,126, entitled “Capacity Maximizationfor a Unicast Spot Beam Satellite System,” filed Aug. 21, 2012, which isa continuation of, and claims priority to, U.S. patent application Ser.No. 13/218,278, entitled “Capacity Maximization For A Unicast Spot BeamSatellite System,” filed Aug. 25, 2011, which is a continuation of U.S.patent application Ser. No. 12/176,629, entitled “Capacity MaximizationFor A Unicast Spot Beam Satellite System,” filed Jul. 21, 2008, whichclaims the benefit of priority to U.S. Provisional Application No.60/951,178, entitled “Capacity Maximization for a Unicast Spot BeamSatellite System,” filed Jul. 20, 2007, the entire contents of each ofwhich are hereby incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to satellite communications systems, andmore particularly to radio frequency communications between a gatewayand a plurality of subscriber terminals via a satellite.

The vast majority of subscribers in urban or suburban areas are servedby either hybrid fiber coaxial, cable, or ADSL networks. Both cable andADSL rely on physical wires to provide network access. The capitalexpenditure depends on the geographic distance between subscribers andaccess nodes. The infrastructure cost is shared by all subscribersresiding in the area. When the subscriber density is low such as in therural or remote areas, the wired infrastructures are too costly to bedeployed. An alternative solution is providing services via satellite.

The satellite is conceptually similar to a base station in a cellularcommunications network, where the base station is located at a very highaltitude above the earth. A geostationary (GEO) satellite is in orbitabout 36,000 km above the equator, and its revolution around the earthis synchronized with the earth's rotation. Therefore, the GEO satelliteappears stationary, i.e., fixed on the earth's surface.

Like a cellular infrastructure, a satellite network can divide thecovered geography (footprint) into many smaller footprints usingmulti-beam antennas. A gateway in the footprint of one spot beam cancommunicate with subscriber terminals in the footprint of other spotbeams. The term spot beam refers to a directional radiation patternprovided by a satellite antenna in which the area of the geographicalcoverage is constrained to a footprint having a direct line of sight tothe satellite. The spot beams can carry two-way communications, sent inpackets at specific time intervals and allotted frequencies. And allwireless technologies for cellular communications such as CDMA, FDMA andTDMA technologies and the combination thereof can also be applied to thesatellite communication. Similar to cellular communication networks thatemploy frequency reuse to maximize bandwidth efficiency, a satellitecommunication system has the additional advantage of employingorthogonal polarization to increase the bandwidth.

A satellite communications system has many parameters to work with: (1)number of orthogonal time or frequency slots (defined as color patternshereinafter); (2) beam spacing (characterized by the beam roll-off atthe crossover point); (3) frequency re-use patterns (the re-use patternscan be regular in structures, where a uniformly distributed capacity isrequired); and (4) number of beams (a satellite with more beams willprovide more system flexibility and better bandwidth efficiency, butrequires more transponders and amplifiers that are in generaltraveling-wave tubes amplifiers (TWTAs). TWTAs are expensive and consumepower that must be supplied on-board the satellite.

The prior art satellite communications systems take the approach ofmaximizing a symbol energy-to-noise-plus-interference (SINR) to theworst-case location within a beam. This approach leads to an increasedcost in subscriber terminals (STs) because the receiver at the STs willbe over-designed to cope with the worst-case condition. Another approachis to divide the available bandwidth into multiple small frequencyranges (different color patterns) and space them apart to reduceinterference. This approach will reduce the available frequencybandwidth for each spot beam and require a large amount of TWTs andTWTAs, therefore require a large power supply on-board the satellite.

Design approaches of prior satellite systems typically do not take intoaccount the effects that various system parameters have on thedata-carrying capacity of spot beams. Indeed, choices made in theselection of particular system parameters may significantly reducecapacity performance, especially in an interference-dominatedenvironment. Thus, there is a need for techniques that allow systemparameter adjustments to be found that will improve data-carryingcapacity.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a method, system, and apparatus forimproved satellite communications. In one embodiment of the invention, asatellite communications system for illuminating a geographic area withsignals comprises at least one gateway, a satellite in orbit configuredto communicate with the at least one gateway and provide a plurality ofservice beams to illuminate a plurality of regions in the geographicarea, and a plurality of subscriber terminals located in the pluralityof regions. The spot beams may include a first spot beam and a secondspot beam. The first spot beam may illuminate a first region within thegeographic area, in order to send information to a first plurality ofsubscriber terminals. The second spot beam may illuminate a secondregion within the geographic area and adjacent to the first region, inorder to send information to a second plurality of subscriber terminals.The first and second regions may overlap.

The first spot beam as sent to at least one of the first plurality ofsubscriber terminals may be affected by interference from other signalsources, including the second spot beam, at a signal-to-interferenceratio C/I. The first spot beam as sent to the at least one of the firstplurality of subscriber terminals may be further affected by noise at asignal-to-noise ratio C/N. Reception of signals from the first spot beamby the at least one of the first plurality of subscriber terminals maybe interference-dominated such that C/I is less than C/N.

Furthermore, the satellite may be operated to maximize data-carryingcapacity of the plurality of spot beams as measured in bits/Hz, byutilizing a beam pattern having a specific number of color(s) offrequency and polarization and specific beam spacing that results inhigher data-carrying capacity of the plurality of spot beams thanachieved with other alternative numbers of color(s) of frequency andpolarization and beam spacings.

According to an embodiment of the invention, the plurality of spot beamsdoes not comprise adaptive coding and modulation (ACM) signals, and thedata-carrying capacity of the plurality of spot beams is maximized bymaximizing minimum data-carrying capacity within the plurality of spotbeams.

According to an alternative embodiment of the invention, the pluralityof spot beams comprise adaptive coding and modulation (ACM) signals, andthe data-carrying capacity of the plurality of spot beams is maximizedby maximizing average data-carrying capacity within the plurality ofspot beams.

In one specific embodiment, the beam pattern has a single color offrequency and polarization, the beam pattern has a beam spacingcharacterized by a cross-over point of less than −6 dB, and the beampattern has a regular frequency re-use pattern.

According to an embodiment of the invention, the first spot beamincludes at least a first portion sent to a first subscriber terminalfrom the first plurality of subscriber terminals utilizing a firstcoding and modulation combination, and the first spot beam furtherincludes a second portion sent to a second subscriber terminal in thefirst plurality of subscriber terminals utilizing a second coding andmodulation combination, the first coding and modulation combinationbeing different from the second coding and modulation combination. Inone specific embodiment, the first coding and modulation combination andsecond coding and modulation combination are selected according to anadaptive coding and modulation (ACM) scheme.

The following detailed description together with the accompanyingdrawings will provide a better understanding of the nature andadvantages of the present invention.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 shows a block diagram of an exemplary satellite communicationssystem according to one embodiment of the present invention.

FIG. 2A shows a diagram of forward links according to one embodiment ofthe present invention. FIG. 2B shows an exemplary modcode tableaccording to one embodiment of the present invention. FIG. 2C shows anexemplary Address-SNR table according to one embodiment of the presentinvention.

FIG. 3 shows a diagram of a prior art three-color spot beam pattern.

FIG. 4 shows a diagram of a two-color spot beam pattern in accordancewith one embodiment of the present invention.

FIG. 5 shows a diagram of a one-color spot beam pattern in accordancewith one embodiment of the present invention.

FIG. 6 shows a diagram of a prior art three-color beam overlap pattern.

FIG. 7 shows a diagram of a two-color beam overlap pattern in accordancewith one embodiment of the present invention.

FIG. 8 shows a diagram of a single-color beam overlap pattern inaccordance with one embodiment of the present invention.

FIG. 9A shows a block diagram of a prior-art satellite having afour-color beam pattern for the service link. FIG. 9B shows a blockdiagram of the satellite 105 having a two-color beam pattern for theservice link according to one embodiment of the present invention. FIG.9C shows a block diagram of the satellite 105 having a one-color beampattern according to another embodiment of the present invention.

FIG. 10 shows a diagram of a forward channel of FIG. 2 in accordancewith one embodiment of the present invention.

FIG. 11 shows a diagram of a one-color beam pattern that usesnon-uniform beam dispersion in accordance with one embodiment of thepresent invention.

FIG. 12A shows a spot beam that uses ACM in various regions havingcircular shape of the spot beam in accordance with one embodiment of thepresent invention. FIGS. 12B and 12C shows a spot beam that use ACM invarious regions having respective hexagon shaped and irregular shape ofthe spot beam in accordance with one embodiment of the presentinvention.

FIG. 13 shows a spot beam having individual subscriber terminalsdistributed among vaguely defined coding and modulation areas inaccordance with one embodiment of the present invention.

FIG. 14 shows a multi-beam forward channel having three parallel datastreams in accordance with one embodiment of the present invention.

FIG. 15 shows a method of implementing adaptive coding and modulationfor maximizing a unicast spot beam capacity in accordance with oneembodiment of the present invention.

FIG. 16 graphically illustrates the Shannon capacity as well as variouswaveform based capacities as a function of the signal-to-noise ratioE_(s)/N₀.

FIG. 17 graphically illustrates an example of the normalized gain|h_(j)|² as function of the location, r, within a beam.

FIG. 18A shows the minimum capacity (bps/Hz) plotted against the amountof beam spacing, for a satellite system operated without ACM, at a“standardized” signal-to-noise (E_(s)/N₀)*=6 dB.

FIG. 18B shows the average capacity plotted against the amount of beamspacing, for a satellite system operated with ACM, at (E_(s)/N₀)*=6 dB.

FIG. 19A shows the minimum capacity (bps/Hz) plotted against the amountof beam spacing, for a satellite system operated without ACM, at(E_(s)/N₀)*=12 dB.

FIG. 19B shows the average capacity (bps/Hz) plotted against the amountof beam spacing, for a satellite system operated with ACM, at(E_(s)/N₀)*=12 dB.

FIG. 20A shows the minimum capacity (bps/Hz) plotted against the amountof beam spacing, for a satellite system operated without ACM, with(E_(s)/N₀)*=0 dB.

FIG. 20B shows the average capacity (bps/Hz) plotted against the amountof beam spacing, for a satellite system operated with ACM, at(E_(s)/N₀)*=0 dB.

FIG. 21A shows the capacity in bps/Hz per beam plotted against thesignal-to-noise ratio E_(s)/N₀, for a satellite system with beam spacingcharacterized by a roll-off value of −3 dB at the cross-over point.

FIG. 21B shows the capacity in bps/Hz per beam plotted against E_(s)/N₀,for a satellite system with beam spacing characterized by a roll-offvalue of −6 dB at the cross-over point.

FIG. 22 illustrates the density (defined as average capacity per unitarea) for satellite systems having different selections of the number ofcolors (L=1, 2, 3, 4, 7).

FIG. 23 presents a four-color system in accordance with one embodimentof the invention.

FIG. 24 provides a summary of different systems having different numberof colors, bandwidth per beam, number of employed gateways, TWT powerper beam, number of TWTs per satellite, the payload aperture, maximumPFD per pole, beam spacing, crossover points, achieved beam capacity,and relative comparison to a four-color baseline system.

DETAILED DESCRIPTION OF THE INVENTION

The ensuing description provides preferred exemplary embodiments only,and is not intended to limit the scope, applicability or configurationof the disclosure. Rather, the ensuing description of the preferredexemplary embodiments will provide those skilled in the art with anenabling description for implementing a preferred exemplary embodiment.It is understood that various changes may be made in the function andarrangement of elements without departing from the spirit and scope asset forth in the appended claims.

Satellite Communication System

Referring first to FIG. 1, a block diagram of an exemplary satellitecommunications system 100 configured according to various embodiments ofthe invention is shown. The satellite communications system 100 includesa network 120, such as the Internet, interfaced with one or moregateways 115 that is configured to communicate with one or moresubscriber terminals 130, via a satellite 105.

The gateway 115 is sometimes referred to as a hub or ground station andservices the feeder links 135, 140 to and from the satellite 105.Although only one gateway 115 is shown, this embodiment has a number ofgateways all coupled to the network 120, for example, twenty or fortygateways. The gateway 115 schedules traffic to the subscriber terminals130, although other embodiments could perform scheduling in other partsof the satellite communication system 100.

Subscriber or user terminals 130 include an outdoor unit (ODU) 134, asatellite modem 132 and an antenna 125. Although the satellitecommunications system 100 is illustrated as a geostationary satellitebased communication system, it should be noted that various embodimentsdescribed herein are not limited to use in geostationary satellite basedsystems, for example some embodiments could be low earth orbit (LEO)satellite based systems. Some embodiments could have one satellite 105,while others could have more satellites working together in concert.

A satellite communications system 100 applicable to various embodimentsof the invention is broadly set forth herein. In this embodiment, thereis a predetermined amount of frequency spectrum available fortransmission. The feeder links may use the same or overlappingfrequencies with the service links or could use different frequencies.The gateways 115 could be placed outside the service beams whenfrequencies are reused.

The network 120 may be any type of network and can include, for example,the Internet, an IP network, an intranet, a wide-area network (WAN), alocal-area network (LAN), a virtual private network (VPN), a virtual LAN(VLAN), a fiber optical network, a hybrid fiber-coax network, a cablenetwork, the Public Switched Telephone Network (PSTN), the PublicSwitched Data Network (PSDN), a public land mobile network, and/or anyother type of network supporting data communication between devicesdescribed herein, in different embodiments. The network 120 may includeboth wired and wireless connections, including optical links. Asillustrated in a number of embodiments, the network may connect thegateway 115 with other gateways (not pictured), which are also incommunication with the satellite 105.

The gateway 115 provides an interface between the network 120 and thesatellite 105. The gateway 115 may be configured to receive data andinformation directed to one or more subscriber terminals 130, and canformat the data and information for delivery to the respectivedestination device via the satellite 105. Similarly, the gateway 115 maybe configured to receive signals from the satellite 105 (e.g., from oneor more subscriber terminals 130) directed to a destination connectedwith the network 120, and can format the received signals fortransmission with the network 120. The gateway 115 may use a broadcastsignal, with a modulation and coding (“modcode”) format adapted for eachpacket to the link conditions of the terminal 130 or set of terminals130 to which the packet is directed (e.g., to account for the variableservice link 150 conditions from the satellite 105 to each respectiveterminal 130).

A device (not shown) connected to the network 120 may communicate withone or more subscriber terminals 130 and through the gateway 115. Dataand information, for example Internet protocol (IP) datagrams, may besent from the device in the network 120 to the gateway 115. The gateway115 may format a Medium Access Control (MAC) frame in accordance with aphysical layer definition for transmission to the satellite 130. Avariety of physical layer transmission modulation and coding techniquesmay be used with certain embodiments of the invention, including thosedefined with the DVB-S2 that is developed in 2003 and ratified by ETSI(EN 302 307), DOCSIS (Data Over Cable Service Interface Specificationdeveloped by Cable Labs) and WiMAX (The Worldwide interoperability forMicrowave Access based on the IEEE802.16) standards. The link 135 fromthe gateway 115 to the satellite 105 is referred to hereinafter as thedownstream uplink 135.

The gateway 115 may use an antenna 110 to transmit the downstream uplinksignal to the satellite 105. In one embodiment, the antenna 110comprises a parabolic reflector with high directivity in the directionof the satellite 105 and low directivity in other directions. Theantenna 110 may comprise a variety of alternative configurations andinclude operating features such as high isolation between orthogonalpolarizations, high efficiency in the operational frequency bands, andlow noise.

In one embodiment of the present invention, a geostationary satellite105 is configured to receive the signals from the location of antenna110 and within the frequency band and specific polarization transmitted.The satellite 105 may, for example, use a reflector antenna, lensantenna, phased array antenna, active antenna, or other mechanism knownin the art for reception of such signals. The signals received from thegateway 115 are amplified with a low-noise amplifier (LNA) and thenfrequency converted to a transmit frequency. The satellite 105 mayprocess the signals received from the gateway 115 and forward the signalfrom the gateway 115 to one or more subscriber terminals 130. In oneembodiment of the present invention, the frequency-converted signals arepassed through a demultiplexer that separate the various receivedsignals into their respective frequency bands. The separate signals areamplified by TWTAs, one for each frequency band and are combined in amultiplexer to form the high-power transmission signals. The high-powertransmission signal passed through a transmit reflector antenna (e.g., aphased array antenna) that forms the transmission radiation pattern(spot beam). In one embodiment of the present invention, the satellite105 may operate in a multi-beam mode, transmitting a number of narrowbeams each directed at a different region of the earth, allowing forsegregating subscriber terminals 130 into the various narrow beams. Withsuch a multibeam satellite 105, there may be any number of differentsignal switching configurations on the satellite 105, allowing signalsfrom a single gateway 115 to be switched between different spot beams.

In another embodiment of the present invention, the satellite 105 may beconfigured as a “bent pipe” satellite, wherein the satellite 105 mayfrequency and polarization convert the received carrier signals beforeretransmitting these signals to their destination, but otherwise performlittle or no other processing on the contents of the signals. A spotbeam may use a single carrier, i.e., one frequency or a contiguousfrequency range per beam. A variety of physical layer transmissionmodulation and coding techniques may be used by the satellite 105 inaccordance with certain embodiments of the invention. Adaptive codingand modulation can be used in some embodiments of the present invention.

For other embodiments of the present invention, a number of networkarchitectures consisting of space and ground segments may be used, inwhich the space segment is one or more satellites while the groundsegment comprises of subscriber terminals, gateways, network operationscenters (NOCs) and a satellite management center (SMC). The satellitescan be GEO or LEO satellites. The gateways and the satellites can beconnected via a mesh network or a star network, as evident to thoseskilled in the art.

The service link signals are transmitted from the satellite 105 to oneor more subscriber terminals 130 and received with the respectivesubscriber antenna 125. In one embodiment, the antenna 125 and terminal130 together comprise a very small aperture terminal (VSAT), with theantenna 125 measuring approximately 0.6 meter in diameter and havingapproximately 2 watts of power. In other embodiments, a variety of othertypes of antennas 125 may be used at the subscriber terminal 130 toreceive the signal from the satellite 105. The link 150 from thesatellite 105 to the subscriber terminals 130 may be referred tohereinafter as the downstream downlink 150. Each of the subscriberterminals 130 may comprise a single user terminal or, alternatively,comprise a hub or router (not pictured) that is coupled to multiple userterminals. Each subscriber terminal 130 may be connected to variousconsumer premises equipment (CPE) 160 comprising, for example computers,local area networks, Internet appliances, wireless networks, etc.

In one embodiment, a Multi-Frequency Time-Division Multiple Access(MF-TDMA) scheme is used for upstream links 140, 145, allowing efficientstreaming of traffic while maintaining flexibility in allocatingcapacity among each of the subscriber terminals 130. In this embodiment,a number of frequency channels are allocated which may be fixed, orwhich may be allocated in a more dynamic fashion. A Time DivisionMultiple Access (TDMA) scheme is also employed in each frequencychannel. In this scheme, each frequency channel may be divided intoseveral timeslots that can be assigned to a connection (i.e., asubscriber terminal 130). In other embodiments, one or more of theupstream links 140, 145 may be configured with other schemes, such asFrequency Division Multiple Access (FDMA), Orthogonal Frequency DivisionMultiple Access (OFDMA), Code Division Multiple Access (CDMA), and/orany number of hybrid or other schemes known in the art.

A subscriber terminal, for example 130-a, may transmit data andinformation to a destination on the network 120 via the satellite 105.The subscriber terminal 130 transmits the signals via the upstreamuplink 145-a to the satellite 105 using the antenna 125-a. A subscriberterminal 130 may transmit the signals according to a variety of physicallayer transmission modulation and coding techniques. In variousembodiments of the present invention, the physical layer techniques maybe the same for each of the links 135, 140, 145, 150, or may bedifferent. The link from the satellite 105 to the gateway 115 may bereferred to hereinafter as the upstream downlink 140.

Referring next to FIG. 2A, a diagram of an embodiment of a forward linkdiagram 200 is shown. A number of gateway antennas 110 respectively havea forward channel 208 through the satellite 105 to a spot beam 204. Anumber of subscriber terminal (ST) antennas 125 are configured in thespot beam 204 to capture the forward channel 208. The ST 130 aredistributed among the n spot beams 204 based generally upon theirpresence within a particular spot beam 204. There are places where thespot beams 204 overlap such that a particular subscriber terminal 130could be allocated to one or another spot beam 204.

The upstream feeder link 140 is separated from the downstream servicelink 150 using some sort of orthogonality, for example, temporal,spatial, frequency, and/or polarization. In one embodiment, the upstreamfeeder link 140 has a feeder spot beam that is geographically separatedfrom the service spot beams, but any type of orthogonality couldaccomplish the separation.

Referring to FIG. 2B, an example of a modulation and coding (modcode)table 202 is illustrated in the form of a block diagram. This form ofmodcode table 202 may, for example, be used by a gateway 115 todetermine the modcode to be used for packets destined for a subscriberterminal operating in a given signal quality range. The table contains acolumn listing a number of modcode formats 205. Each modcode format 205corresponds to a specified signal quality range 210. The signal qualityrange may provide some knowledge on the channel for an associatedregion. For example, the signal quality range 210 can be defined as thesignal-to-interference-plus-noise (SINR) ratio that may be measured atthe subscriber terminals for a predetermined bit error rates (BER)and/or packet error rates (PER) and reported back to the gateway. BERand/or PER can be extracted from a cyclic redundant check (CRC)calculation with the gateway transmits data packets or data framescontaining a certain length of bits whose pattern are known a prior bythe subscriber terminal. Thus, using the signal quality attributed to adestination link for a packet, a signal quality range 210 encompassingthe link may be identified, and the appropriate modcode may be selected.For example, if a destination link has a signal quality within Range 7,the modcode QPSK 3/4 may be used. In some embodiments of the presentinvention, one or more of the ranges may include a reliability margin(which may be beneficial when channel conditions are changing rapidly,for example). One or more of the ranges may be modified dynamically toadjust this reliability margin as well.

In other embodiments of the present invention, other signal qualityindicators may be used, such as a measured signal to noise ratio, anestimated signal to noise ratio, a bit error rate, a received powerlevel, or any other communication link quality indicator. It is alsoworth noting that a number of other data structures may also be used torelate signal quality ranges to modcodes. In one embodiment, each signalquality is associated with a different packet forwarding queue. In stillother embodiments, other information density parameters in addition tomodcode changes may be added to further adapt a signal to environmentalor other conditions.

Adaptive Code Modulation (ACM)

Turning to FIG. 2C, an example of an address/SNR table 250 isillustrated in the form of a block diagram. This form of address/SNRtable 250 may, for example, be used by a gateway 115 to lookup thesignal quality 260 of a subscriber terminal 130 to which a packet isdestined, based on the destination address 255. The tables in FIGS. 2Band 2C may be embodied on one or more memories, which may be either onor off chip, and may be used in conjunction with one another tocorrelate a MAC address with a particular modcode format.

Although a destination MAC address is used in this example, othermechanisms may be used to identify particular subscriber terminals,including destination VLAN-ID, a Destination Internet Protocol (DIP)address, a private addressing ID, any other set of data comprising orotherwise correlated with a destination address. The data address may beparsed from a received data packet after arrival at a device, or it maybe received in any other manner known in the art. It is also worthnoting that a number of other data structures may also be used to relatean address to signal quality.

Once a modcode for a particular packet or packets is identified, forexample using the modcode table 202, it may then be encapsulated, coded,mapped and transmitted in a variety of ways, as known in the art. Oneway to implement an adaptive coding and modulation (ACM) is via theDVB-S2 standard, which specifically provides for its use. As notedabove, ACM may change the modulation format and Forward Error Correction(FEC) codes (modcodes) to best match the current link conditions. Thisadaptation may occur on a frame by frame basis. The discussion thatfollows assumes an IP based packet network in the context of a DVB-S2satellite transmission system, but the concepts may be applied for avariety of systems, including systems implementing DOCSIS, WiMAX, or anywireless local loops (WLLs).

With reference to FIG. 3, a diagram of a prior art three color spot beampattern 300 is shown. These spot beams 304, 308, 312 could correspond tothree different frequency groups, with one group for each color.Patterns with even more colors are also known. The pattern assures thatno directly adjacent spot beams use the same color. Orthogonality isachieved by the use of the different colors. For example, the firstcolor could correspond to 2.0 through 2.1 GHz, the second color couldcorrespond to 2.1 through 2.2 GHz and the third color could correspondto 2.2 through 2.3 GHz. The spot beams are shown as hexagon shaped, butare more circular or oval in shape such that there is overlap betweenthe spot beams 304, 308, 312.

Referring next to FIG. 4, a diagram of an embodiment of the presentinvention having a two-color spot beam pattern 400 is shown. With onlytwo colors available, spot beams 404, 408 will overlap. Here we have arow of spot beams 404 in a first color and a row of spot beams 408 in asecond color. For example, the first color could be 2.0 through 2.15 GHzand the second color could be 2.15 through 2.3 GHz. Along the rows,there will be some confusion between directly adjacent spot beams thathave the same color. By going from three to two colors, the availablefrequency bandwidth for each spot beam 404, 408 increases by fiftypercent.

With reference to FIG. 5, a diagram of an embodiment of the presentinvention having a one-color spot beam pattern 500 is shown. Thisembodiment uses the same or at least partially overlapping frequenciesin each spot beam 504. For example, the spot beams 504 could each use2.0 through 2.3 GHz. All immediately adjacent spot beams 504 use thesame frequency range. Other embodiments of the present invention couldhave patches or portions of the color pattern that have immediatelyadjacent spot beams that use the same or overlapping frequency ranges.

Referring next to FIG. 6, a diagram of a prior art three color beamoverlap pattern 600 is shown. This diagram corresponds to a portion ofFIG. 3, but shows the spot beams 304, 308, 312 as circles rather thanhexagons. FIG. 6 is also idealized in that the overlap could be of anysize as the signal continues outside the circle, but at a lower signalstrength such that the diameter of the circles are somewhat arbitrary asthe radio signal strength falls off quickly with distance relative tothe center. Generally, the STs 130 within the circle can receiveinformation from the spot beam corresponding to that circle.

Overlap occurs in various overlap regions 604, 608, 612. The first typeof overlap region 604 corresponds to an area where STs 130 can receiveboth from a first color beam 304 and a second color beam 308. The secondtype of overlap region 608 corresponds to an area where STs 130 canreceive both from a second color beam 308 and a third color beam 312.The third type of overlap region 612 corresponds to an area where STs130 can receive both from the first color beam 304 and the third colorbeam 312. In the overlap regions 604, 608, 612, STs 130 could optionallyreceive from either spot beam causing the overlap.

With reference to FIG. 7, a diagram of an embodiment of the presentinvention having a two-color beam overlap pattern 700 is shown. Thereare three different types of overlap regions 704, 708, 712 in thisembodiment also. The third overlap region 712 correspond to an areawhere STs 130 can receive from both a first color beam 404 and a secondcolor beam 408. In the first and second types of overlap regions 704,708, directly adjacent spot beams use the same or overlappingfrequencies such that STs 130 in the overlap regions 704, 708 couldbecome confused because of the interference. This embodiment of thepresent invention uses adaptive coding and modulation (ACM) to enablereception in the presence of the interference in the overlap regions704, 708. Effectively, the coding and/or modulation are modified to slowthe data rate until an acceptable signal quality is achieved.

Using ACM, the modulation format and Forward Error Correction (FEC)codes (modcodes) for a data frame may be adapted to better match thelink conditions for each user in a multi-user system. ACM can be used inboth directions. A return channel or other means may be used to reportthe conditions of a receiving terminal. These link conditions are oftencharacterized by the modem's 132 signal to noise ratio (SNR) orsignal-to-interference-plus-noise ratio (SINR) if the modem 132 residesin a color-beam overlap region. In a broadcast system, for example, thedata frame broadcasted to a number of users includes data packetsdesignated only for an individual modem or small group of modems. Amessage transmitted to a user requires fewer symbols and less time whena higher order modulation and higher code rate is used. Lower ordermodulation and lower code rate are more reliable but require more timeto transmit the same size message. Using ACM, each packet may betransmitted at an optimized modulation and coding (modcode) level giventhe destination terminal's link conditions.

With reference to FIG. 8, a diagram of an embodiment of the presentinvention of a single color beam overlap pattern 800 is shown. Here, allbeams use the same or overlapping frequencies in this embodiment. Theoverlap regions 804 receive interference from adjacent beams asfrequencies used are common among beams. Once again, ACM is used tomitigate the effect of interference. STs 130 proximate to the overlapregions may see signals from two or more beams 504. The ST 130 maysample the signal from each beam 504 and use the one that provides themost reliable signal reception. In this way, the system 100 can assignSTs 130 among the beams.

Referring next to FIG. 9A, a block diagram of a prior art satellite 105is shown in block diagram form. The satellite 105 in this embodimentcommunicates with twenty gateways 115 and all STs 130 using twentyfeeder and eighty service spot beams. Each feeder link spot beam feedsfour service link spot beams in this embodiment. Other embodiments coulduse more or less gateways/spot beams. There are likely to be thousandsor millions of STs 130 divided by geography between the service linkspot beams 204. Buss power 912 is supplied using a power source such aschemical fuel, nuclear fuel and/or solar energy. A satellite controller916 is used to maintain attitude and otherwise control the satellite105. Software updates to the satellite 105 can be uploaded from thegateway 115 and performed by the satellite controller 916.

Information passes in two directions through the satellite 105. Adownstream translator 908 receives information from the twenty gateways115 for relay to subscriber terminals 130 using eighty service spotbeams. An upstream translator 904 receives information from thesubscriber terminals 130 occupying the eighty spot beam areas and relaysthat information to the twenty gateways 115. This embodiment of thesatellite only translates carrier frequencies in the downstream andupstream links from the spot beams 308, 304 in a “bent-pipe” fashion,i.e., the only processing is frequency translation and retransmission,but other embodiments could do baseband switching between the variousforward and return channels. The frequencies and polarization for eachspot beam could be programmable or preconfigured.

With reference to FIG. 9B, a block diagram of satellite 105 according toone embodiment of the present invention is shown. This embodiment usestwo colors on the service link spot beams 404, 408. There are eightyservice link spot beams 404, 408. The gateways 115 support the servicelink spot beams with forty gateways 115. With the two colors, eachgateway 115 can support two service link spot beams 404, 408.

Referring next to FIG. 9C, a block diagram of satellite 105 according toanother embodiment of the present invention is shown. This embodimentuses one color on the service link spot beams 504. There are eightyservice link spot beams 504. The gateways 115 support the service linkspot beams with eighty gateways 115. With the one color, each gateway115 can support one service link spot beams 504.

With reference to FIG. 10, a diagram of forward channel 208 according toan embodiment of the present invention is shown. In this simplifiedexample, a superframe 1004 is divided between two modcodes 1008. A firstmodcode 1008-1 is used for the STs 130 largely outside the overlappingregions, and a second modcode is used for the STs 130 inside theoverlapping regions. For example, the first modcode 1008-1 could be 32APSK rate 5/6 and the second modcode could be QPSK rate 1/2. To deliverthe same amount of data, the second modcode 1008-2 uses a larger timeslice of the superframe 1004. The division of time slices between thetwo groups is also affected by the number of group members and thebandwidth requirements of the groups.

Other embodiments of the present invention could have more than twomodcode schemes that divide the data stream (e.g., three, four, five,eight, twelve, sixteen, etc.). The relative size of the modcode schemesin the superframe 1004 can remain static or change over time in variousembodiments. Further, some embodiments of the present invention may notuse a superframe structure and change the coding and modulation asneeded. STs 130 can be moved between the various modcode 1008 as afunction of their bit error rate (BER) or other factors.

Referring to FIG. 11, a diagram of a one-color beam pattern that usesnon-uniform beam dispersion according to one embodiment of the presentinvention is shown. Population gradients 1104 indicate where thepopulation is most dense relative to other gradients in a topographicmanner. In this embodiment, the spot beams 504 can be moved to get morethe central region of each spot beam 504 over the population. Forexample, spot beam 504-1 was moved away from a uniform grid spacing tosit over population 1104-1 more squarely. Other spot beams 504 may alsobe moved. In some cases, there may be two spot beams that overlap to asubstantial degree, for example, at least 80%, 70%, 60%, 50%, 40%, 30%,or 10% overlap.

With reference to FIG. 12A, an embodiment of a spot beam 1200-1 is shownthat uses ACM in various regions of the spot beam. Not in strictadherence to the geometric shapes shown in the figure, the subscriberterminals 130 in the spot beam 1200 are divided among several coding andmodulation (CM) areas 1204. The various CM areas are shown in anidealized shape, but are not completely geometric as STs 130 may beirregularly distributed according to these general areas. As STs 130report higher or lower error rates, they can be moved from one CM areato another. For example, a ST 130 may be assigned to a third CM areabecause of a location adjacent to a neighboring beam causinginterference, but the ST 130 may have a very low bit-error rate (BER).The system may temporarily assign the ST 130 to a second CM area with ahigher data rate and observe the BER. If the BER is acceptable, the ST130 will remain in the second CM area until the BER becomesunacceptable.

The first CM area 1204-1 is generally circular and located near thecenter of the spot beam. A location near the center makes it less likelyneighboring beams in a one- or two-color scheme will overlap. The firstCM area 1204-1 would have the highest data rate by selecting anappropriate coding and modulation, the second CM area 1204-2 would havea lower data rate and the third CM area 1204-3 would have the lowestdata rate. Generally, lower data rates and their corresponding codingand modulation will produce higher link margin or gain. Those areas atthe periphery of the spot beam 1200 are more likely to have interferencefrom neighboring spot beams that can be compensated with the higher linkmargin a lower data rate affords.

Referring next to FIGS. 12B and 12C, two additional embodiments of aspot beam 1200 are shown that use ACM in various regions of the spotbeam 1200. These embodiments demonstrate that other geometries for theCM areas 1204 are possible. The embodiment of FIG. 12B has two hexagonshaped CM areas 1204-4, 1204-5 surrounded by a circular shaped CM area1204-3. In FIG. 12C, the shape of the interior two CM areas 1204-6,1204-7 are irregular and may change over time. Obstructions, geography,weather and other factors may change the geometry.

With reference to FIG. 13, an embodiment of a spot beam 1300 is shownwith individual STs 1308 shown distributed among vaguely defined CMareas 1204. The circular-shaped STs 1308-3 are generally located in anarea defined between an outside the larger hexagon CM area 1204-5 and aninside the circular CM area 1204-3, but there are a few STs that do notfall in that area. As particular STs 1308 no longer need a CMcombination with higher link margin, they are moved between CM areas1204 regardless of the general shape of the CM areas 1204.

Referring next to FIG. 14, a multi-beam forward channel 1400 accordingto an embodiment of the present invention is shown as three paralleldata streams. Each data stream uses a superframe 1004 structure, butother embodiments may not use superframes. Each superframe 1004 is shownwith a first, second and third modcode schemes 1008. These are generallyequal in temporal size in this embodiment, but other embodiments couldvary the size according to the bandwidth usage of the STs 130 that arepart of each modcode scheme 1008. For example, the first modcode scheme1008-1 could be 32APSK 8/9, the second modcode scheme 1008-2 could be16APSK 3/4, and the third modcode scheme 1008-3 could be 8APSK 2/3.Other modcode schemes other than in FIG. 2B can also be used.

The three spot beams corresponding to these parallel data streams may beadjacent to each other. A particular ST 130 assigned with a particularmodcode scheme 1008 will not see the same modcode scheme 1008 used on anadjacent beam in an overlapping way. Other embodiments could havepartial overlap between adjacent beams. Some embodiments could avoidoverlap between some modcode schemes 1008 while allowing more overlap onothers. For example, the center of the spot beam would likely use thehighest data rate, but is also least likely to have any overlap. The CMscheme corresponding to the center of the spot beam could tolerate moreoverlap. The first data packet and at least part of the second datapacket are encapsulated in the same frame. The first and second datapackets use different modcodes because of varying signal qualities. Forexample, the signal quality is likely to be better near the center of abeam as there would likely be less overlap with adjacent beams using thesame frequency.

Referring next to FIG. 15, a process for implementing adaptive codingand modulation in accordance with an embodiment of the present inventionis shown. At process step 1510, a first subscriber terminal (ST)determines a first signal quality range, and a second ST determines asecond signal quality range. The first and second STs may be locatedwithin a first spot beam, or they may be located at the intersection ofthe first spot beam with other adjacent beams. In one embodiment, thesignal quality range may be associated with a signal to noise ratio(SNR) and/or a carrier to interference ratio (C/I) at the VSAT input ofthe subscriber terminal residing at the location r for a predeterminedBER or PER.

At process step 1520, the SNR and/or C/I obtained from the location r issent to gateway 115 via satellite 105 using the return downlink 140. Atprocess 1530, gateway 115 associates a first modcode to the first signalquality range and a second modcode to the second signal quality range.

At process step 1540, gateway 115 receives a first data packet destinedfor the first ST within the first signal quality range and assigns thefirst modcode to the first data packet. At process step 1540, thegateway 115 may also receive a second data packet destined for thesecond ST within the second signal quality range and assign the secondmodcode to the second data packet. At process step 1550, gateway 115encapsulates the first data packet encoded and modulated with the firstmodcode and at least a part of the second data packet encoded andmodulated with the second modcode in a first frame. At process step1560, gateway 115 transmits the first frame to the first spot beam viasatellite 105.

A number of variations and modifications of the disclosed embodimentscan also be used. For example, the gateways could support multiplecolors to reduce the number of feeder links. LEO, GEO satellite orbits,or cellular towers could be used. Further, the invention is not meant tobe limited to only the forward link or only the return link. Someembodiments may use overlapping frequencies on one or both of theforward and return link.

Maximizing Outbound Capacity

According to at least one embodiment of the present invention, thedata-carrying capacity of the service spot beams (referred to asoutbound capacity) as measured in bits-per-second per Hertz (bps/Hz) maybe altered by adjusting certain system parameters. Indeed, the outboundcapacity can be maximized by selecting system parameters appropriately.Such system parameters include the number of “colors” of frequency andpolarization combinations, as well as the amount of beam spacing.

Just as a specific example, a satellite system may have a “bent pipe”feeder link, a single carrier per traveling wave tube amplifier (TWT),417 Mega symbols per second (Msps) time division multiplexing (TDM)outbound, operating at 0 dB output back off (OBO). Such a satellitesystem may specify a beam pattern that uses (1) either 4-color or7-color frequency and polarization combination and (2) an amount of beamspacing defined according to a particular roll-off value at thecross-over point (also referred to as the triple point). These systemparameters may be altered to increase the outbound capacity of thesystem. The frequency re-use pattern may be assumed to be uniformlydistributed. That is, the re-use pattern may be regular in structure.Other system parameters such as number of beams, while they can affectperformance, may not represent a design tradeoff specific todata-carrying capacity. In this example, use of either a 4-color or7-color frequency and polarization combination may result in an outboundcapacity that is not optimized and can be greatly improved. Forinstance, a 1-color frequency and polarization beam pattern may resultin a higher outbound capacity.

Maximization of outbound capacity may be defined in different ways.According to one embodiment of the invention, for a satellite systemthat does not utilize adaptive coding and modulation (ACM), what ismaximized may be the outbound capacity as experienced at the worst-caselocation within a spot beam. This is because in such a non-ACM system,there is only one outbound capacity, and it is determined by aparticular selection of modulation rate and coding that accommodates thesignal quality experienced at the worst-case location within a spotbeam. Here, it is this worst-case outbound capacity that is maximized.For example, this maximization may be achieved by maximizing the ratioof symbol energy to noise and interference, E_(s)/(N₀+I₀), whereE_(s)/N₀ represents the thermal symbol energy to thermal noise ratio,and C/I represents the spot beam carrier to interference ratio.

According to another embodiment of the invention, for a satellite systemthat utilizes adaptive modulation and coding (ACM), what is maximizedmay be the average outbound capacity as experienced at all subscriberterminal locations within a spot beam. This is because in such an ACMsystem, there may be different outbound capacities experienced bydifferent subscriber terminals within the spot beam, resulting fromdifferent selections of modulation rate and coding used at differentsubscriber locations within the spot beam. Here, it is the average ofthese different outbound capacities that is maximized. For example, thismaximization may be achieved by finding the symbol energy to thermalnoise ratio E_(s)/N₀ and E_(s)/I₀ for every subscriber terminal locationwithin the spot beam, then maximizing the capacity averaged over theentire beam. Furthermore, if the user distribution over the service spotbeam is known, it may be worthwhile to maximize a weighted averagecapacity that takes into account the user distribution.

Specific calculations for such “worst-case” and “average” capacity aredescribed in illustrative equations presented below. First, a“standardized” signal-to-noise ratio referred to as (E_(s)/N₀)* isdefined as the signal-to-noise ratio experienced by a subscriberterminal at the beam center, assuming a single-color frequency re-usebeam pattern (L=1):

$\begin{matrix}{\left( \frac{Es}{No} \right)^{*} = {{{SNR}\mspace{14mu} {to}\mspace{14mu} a\mspace{14mu} {terminal}\mspace{14mu} {at}\mspace{14mu} {beam}\mspace{14mu} {center}\mspace{14mu} {assuming}\mspace{14mu} L} = 1}} & (1)\end{matrix}$

This may be a constant value for a particular system. For example, inone embodiment of the present invention, the standard signal-to-noiseratio (E_(s)/N₀)* to a terminal at the beam center for a one-colorpattern (L=1) may be about 6 dB with a 67 cm VSAT.

Next, the signal-to-noise ratio

$\frac{Es}{No}(r)$

and the signal-to-interference ratio

$\frac{C}{I}(r)$

are each defined as function of the location, r, within a beam j:

$\begin{matrix}{{\frac{Es}{No}(r)} = {L \cdot {h_{j}}^{2} \cdot \left( \frac{Es}{No} \right)^{*}}} & (2) \\{{\frac{C}{I}(r)} = \frac{{h_{j}}^{2}}{\sum\limits_{i}\; {h_{j}}^{2}}} & (3)\end{matrix}$

where |h_(j)|² is the “normalized gain,” which is defined as the beamgain relative to the maximum gain. L is the number of color patterns.Here, the signal-to-noise ratio varies linearly with the number ofcolors (L) because it is assumed that the satellite is power limited andthe total power radiated per beam is constant, regardless of thebandwidth of the beam (which is related to the number of colorsemployed). Under this assumption, a system employing 4 colors has abandwidth per beam that is 25% of the bandwidth per beam of a systememploying 1 color. Hence the EIRP density, dBW/Hz, is 4 times as largefor a 4 color system than a 1 color system which results in thesignal-to-noise ratio being 4 times as large for the 4 color system thanthe 1 color system.

Location-specific capacity may be expressed as either the Shannoncapacity or the Waveform based capacity. These two types of capacity caneach be defined as a function of r and can be expressed in terms of anintermediate expression γ(r):

$\begin{matrix}{\left\lbrack {\gamma (r)} \right\rbrack^{- 1} = {\left\lbrack {\frac{Es}{No}(r)} \right\rbrack^{- 1} + \left\lbrack {\frac{C}{I}(r)} \right\rbrack^{- 1}}} & (4)\end{matrix}$Capacity(r)=log₂(1+γ(r)) (Shannon Capacity)  (5)

Capacity(r)=f(γ(r)) (Waveform based capacity)  (6)

where f(γ) is a function that maps signal-to-noise ratio (γ) intocapacity in bps/Hz and is related to the library of waveforms(modulation and codepoint) used in the forward link.

Average capacity can be found by integrating the location-specificcapacity over locations within a beam. This may be defined as:

$\begin{matrix}{{C_{avg}(j)} = {\frac{1}{A_{j}}{\int_{r}^{\;}{{{Capacity}(r)}\ {r}}}}} & (7)\end{matrix}$

where is the area covered by beam j. This represents the averagecapacity over the area covered by beam j.

FIG. 16 graphically illustrates the Shannon capacity as well as variouswaveform based capacities as a function of the signal-to-noise ratioE_(s)/N₀. As depicted in FIG. 16, architectures requiring low FEC coderates generally perform closer to theoretical limits as represented bythe Shannon capacity. For example, the capacity of a waveform utilizingQPSK modulation and DVB-S2 coding is 1.4 dB from the theoretical limit(at E_(s)/N₀ around 0 dB). By contrast, the capacity of a waveformutilizing 8-PSK modulation and DVB-S2 coding is 3 dB away from thetheoretical limit (at E_(s)/N₀ around 7 dB).

FIG. 17 graphically illustrates an example of the normalized gain|h_(j)|² as function of the location, r, within a beam. Here, thenormalized gain is calculated for a system employing a circular aperturewith 10 dB taper in accordance with one embodiment of the presentinvention.

According to an embodiment of the present invention, the capacity of thesystem may be systematically calculated for different choices of systemparameter settings. Both types of capacity may becalculated—“worst-case” and “average” capacity. As discussed previously,“worst-case” capacity may be a more appropriate measure of capacity fornon-ACM systems. “Average” capacity may be a more appropriate measure ofcapacity for ACM systems. The systems parameters that may be variedinclude: (1) the number of “colors” (L) used in the frequency andpolarization beam pattern, (2) the amount of beam spacing, as measuredby the roll-off value at the cross-over point (also referred to as thetriple point). These capacity calculations may also be performed atvarious levels of signal-to-noise (E_(s)/N₀) ratio. By plotting thecapacity of the system as these system parameters are varied, a picturebegins to emerge to indicate how capacity is affected by the choicesmade in different parameter settings. Figures described below representsuch pictures for selected scenarios of system parameter choices.

FIG. 18A shows the minimum capacity (bps/Hz) plotted against the amountof beam spacing, for a satellite system operated without ACM, at a“standardized” signal-to-noise (E_(s)/N₀)*=6 dB. The plot is repeatedfor different selections of the number of colors (L=1, 2, 3, 4, 7).Here, the minimum capacity corresponds to the “worst case” capacityreferred to previously. For comparison, FIG. 18B shows the averagecapacity plotted against the amount of beam spacing, for a satellitesystem operated with ACM, at (E_(s)/N₀)*=6 dB. The plot is againrepeated for different selections of the number of colors (L=1, 2, 3, 4,7). The beam capacity is obtained by multiplying the average capacity(bps/Hz) by the total system bandwidth. For a system bandwidth of 2 GHz,the baseline capacity is about 1 Gbps per beam. In FIG. 18 B, a baselineis chosen as a four-color system. As it can be seen in this figure, aone-color satellite system may have 60 percent more average capacitythan a four-color satellite system, up to the crossover point of about−7 dB.

FIG. 19A shows the minimum capacity (bps/Hz) plotted against the amountof beam spacing, for a satellite system operated without ACM, at(E_(s)/N₀)*=12 dB. The plot is repeated for different selections of thenumber of colors (L=1, 2, 3, 4, 7). For comparison, FIG. 19B shows theaverage capacity (bps/Hz) plotted against the amount of beam spacing,for a satellite system operated with ACM, at (E_(s)/N₀)*=12 dB. The plotis repeated for different selections of the number of colors (L=1, 2, 3,4, 7).

FIG. 20A shows the minimum capacity (bps/Hz) plotted against the amountof beam spacing, for a satellite system operated without ACM, with(E_(s)/N₀)*=0 dB. The plot is repeated for different selections of thenumber of colors (L=1, 2, 3, 4, 7). For comparison, FIG. 20B shows theaverage capacity (bps/Hz) plotted against the amount of beam spacing,for a satellite system operated with ACM, at (E_(s)/N₀)*=0 dB. The plotis repeated for different selections of the number of colors (L=1, 2, 3,4, 7).

FIG. 21A shows the capacity in bps/Hz per beam plotted against thesignal-to-noise ratio E_(s)/N₀, for a satellite system with beam spacingcharacterized by a roll-off value of −3 dB at the cross-over point. FIG.21B shows the capacity in bps/Hz per beam plotted against E_(s)/N₀, fora satellite system with beam spacing characterized by a roll-off valueof −6 dB at the cross-over point.

FIG. 22 illustrates the density (defined as average capacity per unitarea) for satellite systems having different selections of the number ofcolors (L=1, 2, 3, 4, 7). Maximizing the density requires the use ofsmaller beam spacing. One way of achieving smaller beam spacing is theuse of more colors, which results in the reduction of the C/I. As it canbe seen, maximizing density will maximize the capacity into a geographichot spot, which is bigger than 1 spot beam, but provides less overallcapacity.

In one embodiment of the present invention, the total capacity may beoptimized by adopting a single-color frequency re-use beam pattern,along with beam spacing characterized by a roll-off value of less than−6 dB at the cross-over point. Such a single-color system may achieve anoutbound capacity increase of 60 percent over the four-color baselinesystem without an increase in bus power or an increase in bandwidth. A60-beam single-color system with beam spacing of 0.47° (vs. 0.32°) cancover twice the area of the full continental United States of America(FULL CONUS). And the capacity can be increased further by reducingcoverage areas (the capacity can reach about 2 Gbps per beam with thesame coverage area and bus power as the four-color system). However, thesingle-color system does need to increase the payload TWTs from 60×90Watts to 240×23 Watts and requires 40 gateways for a total bandwidth of120 GHz (60 beams×2 GHz).

In some other embodiments, the four-color system may get a bettercapacity density for hot spots when the crossover point is about −2 dB.The four-color system is not very sensitive to crossover points and hasessentially the same capacity for crossover points from −1.5 dB to −7 dB(FIG. 22).

FIG. 23 presents a four-color system in accordance with one embodimentof the invention. This system employs two 45 Watt TWTs per beam for atotal of 90 Watt, the same power TWT used in a known four-color baselinesystem. The four-color baseline system may use two frequency ranges 18.3to 18.8 GHz and 19.7 to 20.3 GHz, each having a 500 MHz bandwidth. Thebaseline system may further polarize the two frequency ranges with aleft and right polarization to achieve the four colors. Due to channellocations, intermodulation products fall out of band. The orthomodetransducer (OMT) combines the signals from the two 45 W TWTs into ssingle waveguide port with the proper waveguide modes at the twofrequency ranges so that a single feed horn may be used to illuminatethe spot beam. The OMT may contribute an 1 dB additional suppressionloss at 0 dB OBO, which is defined as the measured power ratio in dBbetween the unmodulated carrier at saturation and the modulated carrier.

In one embodiment of the present invention, the four-color system isused as the baseline with the following parameters: 2.8 m aperture, beamspacing=0.32°, crossover point=−3.5 dB, and a power flux density (PFD)less than or equal −119 dBW/MHz-m². The single-color system may have thefollowing parameters to cover the same area: 4.1 m aperture, beamspacing=0.32°, crossover point=−8 dB; the bigger reflector will provideabout 3.3 dB more gain, sufficiently more than needed to compensate forthe 1 dB additional suppression loss due to the OBO. There may be twooptions for utilizing this additional gain. Under the first option, theadditional gain is taken as a payload power reduction; for example,reduce TWT power by 2.3 dB. The transmission power will be 2×26 W TWTsper beam (or 3.18 kW vs. 5.4 kW for the four-color system). The beamcapacity is about 1.6 Gbps, and the PFD per pole is less than −125dBW/MHz-m². Under the second option, the additional gain is taken asmore capacity due to the increase of E_(s)/N₀ by 2.3 dB. Thetransmission power is 2×45 W TWTs per beam, the same as the four-colorsystem; but the achieved capacity is now 1.96 Gbps, and the PFD per poleis less than −122.7 dBW/MHz-m².

In another embodiment of the present invention, the single-color systemuses a 3.6 m aperture, beam spacing=0.32°, crossover point=−6 dB; thebigger reflector provides about 2.2 dB more gain, sufficiently more thanneeded to compensate for the 1 dB additional suppression loss due to theOBO. Here again there may be two options for utilizing this additionalgain. Under the first option, the additional gain of 1.2 dB is taken asa payload power reduction; for example, reduce TWT power by 1 dB. Thetotal transmission power of the system is then 60×68 W TWTs. The beamcapacity is about 1.28 Gbps, and the PFD per pole is less than −122dBW/MHz-m². Under the second option, the additional gain is taken asmore capacity due to the increase of E_(s)/N₀ by 1 dB. The transmissionpower is 60×90 W TWTs, the achieved beam capacity is 1.40 Gbps, and thePFD per pole is less than −120.8 dBW/MHz-m².

In another embodiment of the present invention, a two-color system usesa 3.6 m aperture, beam spacing=0.32°, crossover point=−6 dB, and aservice link bandwidth of 500 MHz. The power is 60×54 W TWTs due to the2.2 dB gain of the bigger reflector. The E_(s)/N₀ is 9 dB due to reducedsystem bandwidth; and the PFD is less than −119 dBW/MHz-m². The beamcapacity is about 0.77 Gbps (about 77 percent of the four-color system).One way to increase beam capacity is to increase PFD to −118 dBW/MHz-m²as extra power is available (60×68 W TWTs); the E_(s)/N₀ is 10 dB; andthe beam capacity is 0.82 Gbps (82 percent of the four-color baselinesystem).

In another embodiment of the present invention, the system uses a 4.1 maperture, beam spacing=0.32°, crossover point=−8 dB for a high capacitydesign. The 4.1 m aperture provides 3.3 dB more gain with no additionalsuppression loss. The power is 120×45 W TWTs (the TWT has the same poweras those used in the four-color baseline system). The E_(s)/N₀ is 6 dB(the baseline)+3 dB (bandwidth)+3.3 dB (antenna gain) for a total of12.3 dB. The PFD is less than −118.7 dBW/MHz-m², and the beam capacityis about 1.24 Gbps (24 percent more than the baseline system). Even witha 50 percent power reduction of TWTs (22.5 W TWTs), the achieved beamcapacity is still 1.05 Gbps.

In yet another embodiment of the present invention, the system uses a2.8 m aperture, beam spacing=0.47°, crossover point=−8 dB for a highcoverage area design (to cover the full continental United States. Thepower is 120×45 W TWTs (same as the baseline system). The obtained PFDper pole is less than dBW/MHz-m²; the E_(s)/N₀ is 6 dB (the baseline)+3dB (bandwidth) for a total of 9 dB. The achieved beam capacity is 1.04Gbps. Even with a 50 percent power reduction of TWTs (22.5 W TWTs), theachieved beam capacity of this system is still 0.8 Gbps.

FIG. 24 provides a summary of different systems 2410 having differentnumber of colors 2420, bandwidth per beam 2425, number of employedgateways 2430, TWT power per beam 2435, number of TWTs per satellite2440, the payload aperture 2445, maximum PFD per pole 2450, beam spacing2455, crossover points 2460, achieved beam capacity 2470, and relativecomparison to a four-color baseline system LF4 2480. The relativecomparison is further divided into capacity 2485, TWT power 2486,satellite link bandwidth 2487, and coverage area 2488.

Interference-Dominated Environment

The present invention makes it possible for satellite communications tobe effectively carried out in an interference-dominated environment.Here, the term “interference-dominated” refers to a situation wherereception of signals from a spot beam at a subscriber terminal isaffected by interference from sources that collectively result in asignal-to-interference ratio C/I, as well as noise at a signal-to-noiseratio C/N, such that the signal-to-interference ratio C/I is less thanthe signal-to-noise ratio C/N. That is, C/I<C/N. In more serious cases,C/I may be 3 dB or more below C/N. That is, C/I<C/N−3 dB. Numerousfactors may compound to create such an interference-dominatedenvironment, which has not confronted previous satellite systems. Onefactor may be the high number of service spot beams in the system. Fromthe perspective of a subscriber terminal receiving signals from adesired spot beam, interference may come from not only immediatelyadjacent spot beams, but also from spot beams beyond the immediatelyadjacent spot beams.

For example, referring to the two-color beam pattern shown in FIG. 4, asubscriber terminal in one particular spot beam may receive interferencefrom the six immediately adjacent spot beams in the beam pattern.Variations in interference may exist, as some of these six immediatelyadjacent spot beams may have the same color as the desired spot beam andtherefore cause more interference than those that have different coloras the desired spot beam. In any case, these six immediately adjacentspot beams may not represent the only sources of interference. Thetwelve spot beams just beyond the six immediately adjacent spot beamsmay also introduce interference. These twelve spot beams are fartheraway from the subscriber terminal, but they also contribute to the totalinterference experienced by the subscriber terminal (although to alesser extent). Furthermore, the eighteen spot beams just beyond thetwelve spot beams mentioned above are yet further away but maynevertheless also introduce interference (although to an even lesserextent). In this manner, all spot beams in the system other than thedesired spot beam can potentially contribute, in varying degrees, to thetotal interference received by the subscriber terminal—and together theyincrease the overall interference and thus lower thesignal-to-interference ratio C/I.

Another factor that may contribute to the presence of aninterference-dominated environment is frequency re-use by service spotbeams. For instance, even if two adjacent service spot beams utilizedifferent “colors,” such two colors may operate in the same frequencyrange and only differ by polarization. While appropriate equipment isused to isolate signals of different polarizations, perfect isolationmay not be achieved. As such, to the extent isolation is not complete,interference from service spot beams having a different polarization butthe same frequency range as the desired spot beam may also contribute tofurther increase the overall interference and thus lower thesignal-to-interference ratio C/I.

Yet another factor that may contribute to the presence of aninterference-dominated environment is frequency re-use between feederspot beams and service spot beams. As mentioned previously withreference to FIG. 2, in at least one embodiment of the invention, afeeder link 140 may have a feeder spot beam that is geographicallyseparated from the service spot beams. Such geographic separationtheoretically provides the orthogonality needed to isolate the feederspot beam from the service spot beams. As such, a system according to anembodiment of the present invention may allow the feeder spot beam tore-use the same frequency and polarization as one or more of the servicespot beams. This facilitates more efficient frequency utilization.However, as a consequence of this frequency re-use between feeder spotbeams and service spot beams, a subscriber terminal receiving signalsfrom a desired service spot beam may be affected by interference comingfrom a feeder spot beam found at a geographically separated locationfrom the desired service spot beam. The severity of this interferencevaries depending on how far away the feeder spot beam is located. Suchinterference from feeder spot beams may also contribute to furtherincrease the overall interference and thus lower thesignal-to-interference ratio C/I.

Techniques described above in various embodiments of the invention allowthe satellite communications system to operate in such aninterference-dominated environment. The novel design of satellitecommunication system such as those presented here leads to a significantamount of interference from signal sources within the system.Embodiments of the present invention effectively handle suchinterference levels among spot beams and accomplish efficientcommunication of data within the interference-dominated environment.

Specific details are given in the above description to provide athorough understanding of the embodiments. However, it is understoodthat the embodiments may be practiced without these specific details.For example, circuits may be shown in block diagrams in order not toobscure the embodiments in unnecessary detail. In other instances,well-known circuits, processes, algorithms, structures, and techniquesmay be shown without unnecessary detail in order to avoid obscuring theembodiments.

While the invention has been described with respect to exemplaryembodiments, one skilled in the art will recognize that numerousmodifications are possible. It will, however, be evident that variousmodifications and changes may be made thereunto without departing fromthe broader spirit and scope of the invention as set forth in the claimsand that the invention is intended to cover all modifications andequivalents within the scope of the following claims.

What is claimed is:
 1. A satellite for illuminating a geographic areawith signals, the satellite comprising: a transmission subsystemoperable to generate a plurality of service link signals; a spot beamantenna system coupled to the transmission subsystem and operable toemit to create a plurality of service link spot beam signals each in acorresponding spot beam, wherein: each of the plurality of spot beams isoperable to illuminate a distinct geographic area when the satellite isplaced into orbit, and portions of a first one of the plurality of spotbeams overlaps a second one of the plurality of spot beams so that asignal-to-interference ratio of a first one of said service link signalsin the first one of said plurality of spot beams isinterference-dominated by interference caused by a second one of theservice link signals in the second one of the plurality of spot beams.